With Kyrie Irving gone, the Celtics will be counting on 21-year-old Jayson Tatum, left, and Jaylen Brown, who turns 24 on Thursday, to step up this season. AP Photo/Chris Szagola

The Boston Celtics have pushed the idea of a fresh start since free agency, with Kemba Walker filling the tarnished hole left by Kyrie Irving, and lord knows who jumping into the chasm left by Al Horford.

SEASON OPENER

WHO: Boston Celtics at Philadelphia 76ers

WHEN: 7:30 p.m. Wednesday

TELEVISION: ESPN, NBCSB

Underachievement replaced by, if Brad Stevens succeeds after a down year, overachievement – for that’s the only way the Celtics go far this season.

Marcus Smart is now trying to be the resident unifier, and he’s looking for simple changes.

“Everybody’s determination. We’re all out here playing hard,” the Celtics guard said of what he’s noticed. “We’re willing to do the little things. That’s big for a new team, with a lot of new faces, coming into a new season and trying to get terminology, concepts down. The young guys have really stepped up and been accountable, and we’ve held everybody accountable, so it’s good to see everybody going through these workouts.”

The Celtics are banking heavily on the rising stars of Jayson Tatum and Jaylen Brown this year after what, especially in the case of Tatum, was a season that went sideways. Stevens will be forced to take a committee approach in the paint, and Walker will have the opportunity to feed off talent beyond his own, for a change.

The Celtics have a number of checkpoints to reach if they’re going to rebound from last season’s downgrade. Here are five of them:

THEY’RE NOT KIDS ANYMORE

Tatum was noodged by an ESPN article entitled, “The de-Kobe-ing of Jayson Tatum has begun,” that explained how the Celtics forward is trending away from the mid-range candy he ate up from Kobe Bryant during their workouts two summers ago.

Though he’s clearly placed a premium back on his 3-point shooting and rim finishes, Tatum insisted that he was going to continue taking mid-range shots – at least good ones, which some analytics types believe don’t exist. But if Danny Ainge’s grand vision for this scoring forward is to get back on track, the time is now for Tatum to take on a more veteran role, and not just as a scorer. The good news here is that Stevens has praised Tatum’s growth defensively.

That’s pivotal, because as a high-scoring forward, Tatum is going to guard players like himself almost every night.

Brown, unlike Tatum, pulled out of 2018-19 on the upswing, both as a finisher and in the defensive end, with an ever-improving 3-pointer.

But without that Horford/Irving umbrella, both of these players are now veterans, by virtue of experience if not yet age. Boston’s fortune depends on both living up it.

HAYWARD’S JOURNEY BACK

Clippers star Paul George once told Gordon Hayward that it takes two years to recover from the kind of gruesome leg fracture that both players suffered. George spoke from experience, after returning from his 2014 injury to average what at the time was a career high 23.7 scoring average in 36 minutes per game during the 2015-16 season. Hayward is entering Year 2 now, in the third year of a max contract that so far hasn’t given the Celtics much return.

Hayward, of course, now has his chance to reverse that history.

If the Celtics are to go anywhere this season, Hayward will need to take over the offensive role that Horford used to fill. The offense should thrive when it goes through Hayward’s hands as a surrogate point guard, enabling Kemba Walker to do more work and scoring off the ball. Though Hayward’s 3-pointer came through in stretches last season, it will be helped more now by a more consistent burst to the basket and confidence from the knowledge he’s physically healed. His fade during the conference semifinals against Milwaukee appeared to be more a matter of confidence than what he could do physically, and Hayward spent his summer in Boston working early in the morning without worries about his health.

He shot 39.8 percent from 3-point range and averaged a career-high 21.9 points his last season in Utah. No one’s predicting he’ll score at that clip again. But if his 3-pointer comes even close to that kind of efficiency, the Celtics’ offense will open like the doors of Oz.

KEMBA AS THE LEADER

Walker is one of the most coach-friendly stars in the business. Where Kyrie Irving was enigmatic and single-minded with how he chose to function under Stevens, the new Celtics point guard is as non-confrontational as they come. He jumped at the opportunity to join the Celtics. It freed him from eight years of being the only real talent on the Charlotte roster, and thus doomed to carrying the Hornets on a nightly basis. No one else was going to do it.

Here he can play off the ball, find better shots, and thrive in a free-flowing offense – conditions that could lead to a great statistical season for the guard.

Also unlike Irving, he won’t attempt to co-opt leadership. Irving failed when he tried, and Walker likely won’t force himself on the others. The Celtics already have a vocal and, if necessary, confrontational presence in Smart. Walker is more comfortable in the Horford role – leading by example. He’ll be able to be himself in this setting.

DEFENDING THE PAINT

Barring some kind of trade down the road, the Celtics will patrol the paint with a committee this season.

Daniel Theis should be an effective small ball center, and he’ll be the best they have at guarding all of those so-called stretch fives. It’s also hard to see him as the starting option on opening night, against Joel Embiid, without some kind of zone or continuous help mixed in.

Enes Kanter is the best rebounder the Celtics have had in memory, and he’ll command attention in the post with what appears to be a second unit role. He’s also a defensive liability. Robert Williams may eventually be the starter – he easily has the most defensive upside – but right now he’s playing like what he is, a second-year big man who still needs a lot of time and savvy. Vincent Poirier may be the most mature – or at least the most defensively balanced – big man on the roster. Stevens will spend a lot of time adjusting his way through this group.

YOUTH COULD BE SERVED

As evidenced by the preseason, Grant Williams has much to learn about switching and defending NBA bigs, but his overall maturity and poise are developed enough for a spot in the rotation.

Carsen Edwards can get off his shot against anyone. There may not be another player on this team who can check in off the bench and reach maximum heat with one touch of the ball. The fact that both played three seasons of college ball actually pays a dividend here. The league tends to place a lower ceiling on that kind of player. But both are ready to contribute now.